In recent years, increasingly intensive effort has been devoted to systems for improvement of air and water quality. For the most part, those efforts are concerned with control of emissions from facilities created by man, possibly because these are regarded as the only sources of contamination subject to effective control at acceptable cost. One of the sources of air contamination which has been given attention is emission of NO.sub.x generated by fixation of atmospheric nitrogen in high temperature zones of combustion processes such as in internal combustion engines and fired furnaces. As noted above, two of the oxides of nitrogen combine with water to form corrosive acids; all the oxides of nitrogen are suspected of contribution to photochemical smog.
Several methods for reducing NO.sub.x emissions have been investigated and reported. It has been proposed that these air contaminants be reacted with or absorbed by liquid or solid agents, thus posing a disposal problem with respect to the spent reagent. It has been shown that NO.sub.x can be reacted with various reducing agents including carbon monoxide, hydrogen and ammonia to leave innocuous agents for discharge to the atmosphere. For example, ammonia reacts with NO.sub.x under proper conditions to yield nitrogen and water which can be freely discharged without adverse effect.
It has been demonstrated that the transition metals are catalysts for conversion of NO.sub.x by reaction with reducing agents such as carbon monoxide and ammonia. Patent application Ser. No. 340,809 distributed by National Technical Information Service of the U.S. Department of Commerce describes the use of ion exchanged zeolite, specifically mordenite, as catalyst for reduction of NO.sub.x by carbon monoxide. Thomas and Pence reported reduction of NO.sub.x with ammonia catalyzed by a zeolite in a paper entitled "Reduction of NO.sub.x with Ammonia over Zeolite Catalysts" presented at the Air Pollution Control Association Meeting at Denver, June 10, 1974. The specific catalyst employed by Thomas et al. was hydrogen mordenite.
Those zeolite catalysts have been used in the form of extrudate pellets in beds which produce substantial pressure drop. In fixed installations such as fired furnaces, the pressure drop through the bed constitutes a reduction in draft applied by the stack. In mobile equipment, automobiles and the like, the bed pressure drop applies a back pressure to engine exhaust which tends to reduce efficiency of the internal combustion engine. A like effect of pelleted platinum catalyst for oxidation of carbon monoxide and unburned hydrocarbons in automative exhaust was obviated by applying the metal to "monolithic honeycomb" structures of refractory support material. Metals on monolithic honeycomb have also been proposed for reduction of NO.sub.x. Similar application of zeolites to the surface of honeycomb refractory raises problems of abrasion of the zeolites, which are relatively fragile as compared to platinum by gases and entrained fly ash or other solids.
British Pat. No. 1,441,448 contemplates the use of a grid work of honeycombs superficially coated with zeolite crystals as a catalyst, especially for cracking of petroleum hydrocarbons. The patent does not disclose catalytic composites in which zeolite is embedded or disseminated throughout a porous matrix that permits access to the active zeolitic crystals while minimizing losses of the crystals due to attrition during use or handling.
The synthesis of zeolites from calcined clays, especially kaolin clay, is known. For example, it is well-known that metakaolin (kaolin clay calcined at a temperature of about 1200.degree. to 1500.degree. F.) will react with sodium hydroxide solution to produce sodium zeolite A. It is also known that when kaolin is calcined under more severe conditions, for example 1700.degree. to 2000.degree. F., it will react with sodium hydroxide solution, small amounts of metakaolin preferably being present, to synthesize faujasite-type zeolites useful in hydrocarbon conversion processes. Reference is made to the following commonly assigned patents of Haden et al.: U.S. Pat. Nos. 3,335,098, 3,338,672, 3,367,886, 3,367,887, 3,391,994, 3,433,587, 3,503,900, 3,406,594, 3,647,718, 3,657,154, and 3,663,165. In the processes of these patents synthetic faujasite-type zeolite is crystallized either as a pulverulent mass or as composite fluid or pelleted particles.
In accordance with the teachings of U.S. Pat. No. 3,119,660 to Howell et al., preformed metakaolin or preformed mixtures of metakaolin and zeolite A were reacted with caustic to form 100 percent zeolite A. By adding sources of soluble silica to the reaction mixture, zeolite X or zeolite Y was formed as a constituent of pellets or the like.
U.S. Pat. No. 4,007,134 to Liepa et al. deals with the use of extruded zeolitic honeycombs in the carbonation of soft drinks. The honeycombs contain over 40% zeolite and are prepared by extruding preformed zeolitic molecular sieve crystals, preferably with a known binder such as clay, and calcining the resulting extrudate to harden the structures. It is well-known that zeolitic molecular sieve crystals lack thermal stability when they are calcined at high temperatures. While high temperatures favor hardening of clay binders, the presence of the zeolite in the unfired honeycomb structures of U.S. Pat. No. 4,007,137 precludes the use of high temperatures, e.g., 1700.degree. F. or above. Temperatures of this order are needed to secure the high strength required for many uses of catalysts and catalyst supports. This process limitation is obviated by practice of the process of the instant invention.